The present invention is directed to an apparatus and system for measuring a physical property, such as thickness, of a sheet material. The present invention is more specifically directed toward a caliper gauge for measuring the thickness of a sheet of paper which is in the process of being manufactured by a papermaking machine and, therefore, is moving at a high speed through the caliper gauge.
Various types of caliper gauges are known in sensor technology and are used for measuring the thickness of rapidly moving sheet material. One type of caliper gauge is called a "contacting caliper gauge". Contacting caliper gauges typically have two opposing pads which are forced into contact with opposite sides of the sheet. The distance between the pads is measured and directly related to sheet thickness or "caliper".
Under some situations, however, contacting caliper gauges may suffer from certain shortcomings. For example, United States paper manufacturers have formed an association (TAPPI) to promote uniform standards for the paper industry. Since paper is somewhat compressible, the TAPPI standard for measuring paper sheet caliper requires that the measurement be based upon results obtained from a contacting caliper gauge with a certain specified pressure exerted by the opposing pads on the paper sheet. Unfortunately, however, the requirement that a caliper gauge contact the sheet under pressure poses problems when the caliper of particularly lightweight, thin or weak paper is being measured. Under a fixed pressure, such sheets are prone to being torn by the sheet-contacting pads. This is particularly true when an imperfection or other portion of the sheet which abruptly increases in thickness is drawn rapidly through the caliper pads.
Another problem with such contacting caliper gauges is caused because modern paper mills manufacture paper sheet at high sheet speeds which can approach 60 MPH. Such a rapidly moving sheet drags a boundary layer of air along with it such that, near the surface of the sheet, the boundary layer of air is moving at the same speed as the sheet. As the sheet passes between the opposing caliper pads, an "air bearing" effect is created which tends to force the pads away from the surface of the sheet. In this situation, the pads are said to be "flying above" the sheet. Thus, at high paper speeds, the pads of a "contacting" caliper gauge may actually fail to contact the sheet. However, as previously mentioned, conventional caliper gauges determine sheet thickness based upon the measured distance between opposing pads. Thus, the flying effect can induce an erroneous caliper measurement by making the sheet appear thicker than it actually is. The flying problem increases as the sheet speed, and hence the speed of the boundary layer air, increases. In fact, conventional "contacting" caliper gauge designs tend to become airborne by up to about 40 microns at higher sheet velocities. Obviously, this is unacceptable when attempting to measure sheet caliper to within 1 micron accuracy under a wide range of sheet velocities.
Previously, caliper designers have given little, if any, consideration to aerodynamics in the design of the caliper pads. For example, certain prior caliper pads have been essentially disk-shaped, with smooth, rounded pad edges to avoid snagging the sheet. These pads tended to fly above the sheet. To avoid flying, the force on these pads was simply increased. However, increasing the force on the contacting caliper pads to counteract the flying effect has not proved to be a satisfactory solution. Increasing the force on opposing pads will tend to counteract the flying effect resulting from the fast moving boundary layer air, but this increased force also increases the tendency of the pads to tear the sheet. Accordingly, the present inventors have recognized the need for a contacting caliper gauge which will remain in contact with the sheet surface (or fly only a very small distance off of the sheet surface) under relatively little external force, while having little or no tendency to tear the sheet.